Radiative Properties of Cirrus Clouds in the Infrared Region

A multiple-scattering radiative transfer model is employed to evaluate the 11 μm and the broad-band infrared (IR) fluxes, cooling rates and emittances in model cirrus clouds for a number of standard vertical atmospheric profiles of temperature and moisture. The single-scattering properties for scattering by mono- and polydispersed randomly orientated long ice columns and for the associated polydispersed equivalent spheres are used in the calculation. The results reveal IR reflectance at the cloud base of 4% (spheres) and 6% (cylinders). This reflectance modifies significantly the cloud effective emittances, cloud cooling rates and the emission by the total atmospheric column. It is shown that the radiative properties of model cirrus clouds determined under the equivalent sphere approximation represents well the properties determined for scattering by randomly orientated columns. The largest difference between the sphere and cylinder models is for reflectance which is a function of the degree of anisotropy of the scatter. It is shown that the relative contribution to the downward radiative flux at the cloud base (and within the cloud) varied according to the temperature differences between the cloud and the effective radiative temperature of the warmer atmosphere below the cloud. For example, the reflection contribution to the downward effective emittance varies for cylinders (spheres) from 25% (15%) of the total effective emittance in a model tropical atmosphere to 10% (5%) in a subarctic summer model atmosphere. The existence of IR reflectance from high clouds may account for the previously reported discrepancies between the broad band (effective) emittances derived from observed flux profiles and the theoretical values. The results suggest that the reflectance of high clouds should definitely be included in any parameterization scheme. It appears that the effective emittance is not as useful for high-cloud parameterization as for low-level cloud because of its more pronounced dependence on the temperature structure of the atmosphere.